The present invention relates to a valve assembly for recirculating exhaust gases in an internal combustion engine, and more particularly, to such an assembly which combines an exhaust gas back-pressure transducer and an EGR (exhaust gas recirculation) valve.
The use of EGR valves generally involves the recirculation of the exhaust gases at a rate approximately proportional to the rate at which air flows into the engine. Such valves have been made responsive to the vacuum in either the intake manifold or the carburetor or, in some cases, have been made responsive to the position of the throttle.
An example of a prior art combination EGR valve and exhaust gas back-pressure transducer is illustrated in U.S. Pat. No. 3,756,210. The reference patent illustrates a valve assembly in which exhaust gas back-pressure (positive gauge pressure) is applied against a diaphragm connected to a slide valve, so that when the pressure reaches a predetermined level, the diaphragm moves, sliding the valve to the open position. The opening of the valve permits vacuum from the intake manifold to enter a chamber which is generally at atmospheric pressure. The incoming vacuum reduces the pressure within the chamber, causing the movement of a second diaphragm connected to the EGR valve member. The reduced pressure moves the diaphragm, opening the EGR valve and permitting exhaust gas to flow to the intake manifold through the passages used to communicate vacuum to the valve assembly.
Because of the use of a common passage for both the vacuum and the recirculated exhaust gas, the valve assembly of the cited reference may tend to oscillate rapidly between the "open" and "closed" positions, because each time the EGR valve opens, the vacuum needed to keep it open is suddenly mixed with exhaust gas at a positive pressure. In the prior art device described, such oscillation is especially undesirable because the valve utilized to permit the passage of vacuum into the control chamber must rapidly slide back and forth between the "open" and "closed" valve positions. The resulting sliding friction represents an operating load within the valve assembly, which tends to cause erratic and less reliable operation and necessitates larger and stronger diaphragms, biasing springs, and other members.